This invention relates generally to subsurface well apparatus, more specifically, to apparatus for perforating subsurface earth formations, and particularly to pressure actuated apparatus for firing tubing conveyed perforating apparatus.
It has become common practice in the completion of oil and gas wells to perforate the well casings and the surrounding formations to bring a well into production. One method of providing this capability has a perforating apparatus attached to the end of a tubing string which is lowered and set in place when the perforating apparatus is opposite the formation to be produced. The perforating apparatus may then be detonated and the well placed into production through the tubing strings. One example of a tubing conveyed perforating apparatus can be found in U.S. Pat. No. 4,523,643, issued to J. R. McGlothen, and assigned to the assignee of the present invention, which is incorporated herein by reference.
The systems for firing the perforating devices have typically been either an electrical firing system or a non-electrical percussion firing system activated by dropping a member through the tubing. Neither method has been entirely satisfactory in the past. Electrical firing systems require care in connecting and running and can be activated from stray electrical currents. In addition, electrical connections can be short-circuited by moisture. Percussion firing systems commonly require a bar member, referred to as a "go devil", be dropped through the tubing string thereby impacting a percussion firing assembly. These percussion firing assemblies typically have some primary explosives in the perforating apparatus while it is affixed to the tubing and lowered into position within the well. As a result of the deficiencies of these systems, accidental and premature firings are a possibility. In addition, in a highly deviated well the bar member may not reach the perforating gun.
In an effort to overcome some of the deficiencies of the electrical and bar activated firing systems, various pressure responsive firing systems have been developed. In one typical embodiment an increased pressure is applied through the tubing string to the pressure responsive firing assembly. The increased pressure forces a firing pin into contact with an explosive detonator, thereby firing the perforating device. One such system is illustrated in U.S. Pat. No. 2,304,408, issued to A. J. Holifield. Systems of this type are unsatisfactory where the perforating operation is to be conducted under conditions of a pressure "underbalance", where the borehole pressure is less than formation fluid pressure. Such underbalance perforating operations have become common in the area of tubing conveyed perforating.
These and other disadvantages are overcome with the present invention by providing method and apparatus for firing subsurface perforating apparatus using a pressure actuated firing system for use when perforating in an underbalanced pressure condition.